I have a road racing car, competing in the Camaro/Mustang Challenge series. The car is completely stripped and I use a Painless harness, like what would be used in a street rod.

Sorry for the length, but I wanted to outline the steps and symptoms I saw

The car is a '96 LT1 M6 OBD2. I have no easy way to scan for codes. The COMM port is sealed by rule and would require a redyno cert if the seal is broken. Also, I don't have 12V applied to the port to power the Autotap module. Both are not big hurdles, but the recert on the dyno is 200 mile round trip on the trailer...

In August, while at the track, I got some rear wheel brake hop going into a turn.

When I went to roll on the gas, the car fell on it's face. After a tow back to the garage, there was no fuel pressure.

Jumped trough hoops getting another fuel pump installed

BTW ... a 3rd gen fuel pump will work. Just CAREFULLY pry apart the cannister and use SS hose clamps to hold it back together.

Still didn't fix it.

Applied 12V directly to the pump, and all was good. Applied 12V directly to the relay and all was good. So, the signal from the PCM to tell the FP to run was missing. I finished the weekend by installing a toggle switch on the dash that sends 12V to the FP test circuit on my Painless harness

Got back home and installed a spare PCM and all was good.

For laughs, I reinstalled the 1st PCM and all was good ... hmmm.

At this point, I figured something went haywire and just needed a computer reset. Every time I moved the car or started it, everything worked as it should. So, I left PCM #1 installed.

Went back to the track this weekend. And no, I didn't take PCM #2 with me ...

After about 4 or 5 laps, the fuel pressure died once again. Same symptoms. Damn!

So, I flipped the FP switch and all was good. After session, I parked the car as normal.

Next session was called ... no ignition spark!

I thought about the computer reset, so I turned off all power via my master disconnect. After several attempts and various times with the power disconnected, still no spark.

I removed the PCM to check for a bad connector ... all the normal stuff.

I plugged PCM #1 back in and spark AND FP returned! I took it down the access road and after about a minute of running, the FP died once again. I flipped the 12V to FP switch, turned around and made it back to my pit.

I surmised that it was a cascading of internal events and that spark was next on the list of things to die. I am at the track, 300 miles from PCM #2 ...

Through some creative phone calling, I was able to get my wife to shuttle PCM #2 to a friend's wife who was coming to the track for the weekend. PCm #2 was delivered first thing Saturday morning.

I installed it and had no further issues all weekend long.

However, something is different.

Using PCM #1, when working properly, would build and hold fuel pressure. Over time, it would leak down, but the fuel pressure was always there. The car would start immediately.

Now, it has to spin the motor a few revolutions before starting. It does not build and hold fuel pressure as it once did, with either PCM.

I hear the fuel pump come on and the relay click, but it just doesn't build presure. Could this be a fuel pressure regulator and coincidental to my PCM issues? Or is there some other, biger isuue I'm just not seeing ...

As far as the PCM issue go, I looked for signs of damage, heat exposure ... the normal stuff. It all looked just fine ... even the aluminum mount I made for the PCM from relocating the PCM to inside the cockpit, in the passenger foot well. No signs of heat or damage

Can anyone share some insight as to what the cause might be? Is this just one of those things that sometimes happen?

 

Anyone?

 

Using PCM #1, when working properly, would build and hold fuel pressure. Over time, it would leak down, but the fuel pressure was always there. The car would start immediately.

Now, it has to spin the motor a few revolutions before starting. It does not build and hold fuel pressure as it once did, with either PCM.

I hear the fuel pump come on and the relay click, but it just doesn't build presure. Could this be a fuel pressure regulator and coincidental to my PCM issues? Or is there some other, biger isuue I'm just not seeing ...


Are you saying you can make that pump work with the switch everytime it quits?

Your PCM doesn't know what pressure you are running, sounds to me like a faulty pump. I would think if the regulator disk burst, you would have gas coming out of the vacuum line attached to the regulator. I wouldn't think that would "repair itself" and work properly.

When my pump went out, actually just the check valve went bad. It would make pressure and immeadiately lose it. I would turn the key on...the pump primed and pressure was gone, that fast.

Are you saying you can make that pump work with the switch everytime it quits?

Mike

 

Yes ... If I apply 12V to the pump, it will pump and hold ~40psi. Turn thepump off and the pressure will leak down almost immediately.

Before, it would hold the pressure, sometimes for days on end.

And it's not the pump. I thought the same thing. The pump was replaced first thing.

 

There are 2 things that control pressure. The regulator and the check valve in the pump. Prime the pump and shut off the return line, if it holds it's your regulator. If not it's your pump.

By the way, did I mention, my pump was a brand new walbro 255l/hr and the check valve was bad. So just because it's a new pump don't rule out that it is good.

Try shutting off the return line and see if it holds pressure, as that is the eaisest fix

Mike

 

Let's say it is the check valve ...

How would that affect fuel pressure when powered from the PCM? Or am I chasing two seperate issues?

 

My bad...I didn't read your first statement :

"If I apply 12V to the pump, it will pump and hold ~40psi. Turn thepump off and the pressure will leak down almost immediately"

That makes no sense at all....so let me make sure I understand what you are saying. When you put 12 v to the pump...it runs the whole time....doesn't just prime and quit correct?

 

Correct ... I am by-passing the relay and PCM and have routed a circuit that applies 12V constantly.

If I'm not abundantly clear, this is on a road racing car. There is no stock wiring harness. I have a Painless wiring harness and it worked flawlessly for 4 months.

The only thing that is repeatable is that when the PCM stops sending the 12V supply to the fuel pump relay, I can completely unplug the PCM from the wiring harness, re-plug it back in and the PCM will send the appropriate signal to the relay ... for a while.

The longer the PCM sits without power, the longer it will control the relay.

PCM #2 works as it should ... NOT using the 12V by-pass switch I wired in. It sends the 12V signal to the relay as it should.

The pressure issue surfaced at the same time. I am beginning to think it is a coincidence and not related to the voltage.

 

If it works as it should with PCM #2, there is obviously something wrong with PCM #1. Why or what went wrong with it, I can't tell ya.

Are you saying it will hold pressure with PCM #2 also? Or that it sends voltage to the relay like it is supposed too? If it holds pressure with PCM #2, but not #1, your car is possessed. J/K

Mike

 

I know it's a confusing sequence of events ... and the car may very well be a Dodge Demon in disguise ... I owned one those. I never want to see an AFB carb again as long as I live ...

At the time PCM #1 began shutting off voltage to the FP relay, the fuel system would not hold pressure once the fuel pump was shut off.

As long as the fuel pump was running, by whatever means, it would hold proper fuel pressure.

Once the voltage was removed, by whatever means, fuel pressure would bleed off immediately.

Before the original issue with PCM #1 creeped in, it would hold residual fuel pressure even after the system was shut down.

That is why I am leaning towards seperate, coincidental issues ...

The PCM problem has been fixed ... I hope as only track time will tell. I can't drive it on the street ...

That leaves the residual fuel pressure issue which does sound like a check valve. Now my question is ...

Does a 3rd gen fuel pump have a check valve?

I was strapped for time as I had already mised 2 races. So, I cracked open the canister that houses the fuel pump itself, removed the pump, went to a local O'Reilly's, pulled a 3rd gen pump, compared the 2 side by side and they appeared to be identical. So I installed a 3rd gen pump inside the canister, buttoned it all back up and went racing ...

 

To answer your question about the 3rd gen having a check valve, Yes it does! I am almost positive( not 100% sure) that they are the same pump. A buddy of mine has a 3rd gen and although we didn't have our pumps side by side, I remember them the same.

What did you do to the computer to fix the voltage problem?

Mike

 

To fix the voltage problem with PCM #1, I would completely unplug it. And depending upon how long that was, seemed to directly affect how long it would act "normal".

But, to completely fix the problem, I replaced PCM #1 with PCM #2.

The fuel pressure bleed off remained.